Power is delivered to the pulley from an electric motor via a gearbox and rotation of the pulley tends to entrain the cord, thus operating the blind. However, to ensure that the cord is reliably gripped by the pulley, it has hitherto been found necessary to ensure that the cord is tensioned in its passage around the pulley. Clearly, once the pulley begins to drive the cord, there will be a certain amount of tension in the cord upstream of the pulley, but it has been found that tension (additional to that resulting from the weight of the cord itself) is required both upstream and downstream of the pulley to ensure that as the pulley begins to rotate, it takes the cord with it. This will be referred to as "back-tensioning" of the cord. It gives rise to excessive wear on the pulley bearings and increases the drag of the mechanism, thereby increasing the motor current required to drive the blinds
Furthermore, the task of assembling the driving mechanism of the blind requires a degree of dexterity, since the cord must be kept properly positioned relative no the pulley until the cord can be tensioned to keep it in place.